American Petroleum Institute

Prevention of Brittle Fracture of Pressure Vessels
 N RP 920

Annotation

2.3 Scope

2.3.1 VESSELS

The pressure vessels covered in this recommended practice are primarily the welded vessels constructed in accordance with Section VIII, Division 1, of the ASME Code or with the API Code for Unfired Pressure Vessels, which was discontinued in 1956.

2.3.2 MATERIALS

The pressure vessel steels of primary concern are the nonimpact tested steels (except for bolting) included in the ASME Code under Part UCS of Section VIII, Division 1. Before the 1987 Addenda, steels of this kind were permitted by Section VIII, Division 1, of the ASME Code for a minimum temperature of ?20°F without restriction; they were also permitted for lower temperatures caused by ambient temperatures. (See Appendix B for information on obsolete specifications.)

In considering the toughness properties of welded base metals, attention should be given to possible lower toughness in the heat-affected zone (HAZ) and to the possible formation of HAZ cracks. Users may compensate for the reduction in toughness by raising base metal requirements. The chance of brittle fracture originating from cracks in the heat-affected zone can be reduced by increased inspection of the zone. (See Appendix C for guidance on the inspection of HAZ cracks.)

2.3.3 SERVICE EMBRITTLEMENT

This recommended practice does not cover embrittlement that can occur in service and result in loss of toughness. One example is temper embrittlement that may occur in 1?Cr-?Mo, 2?Cr-1Mo, and 3Cr-1Mo steels at elevated temperatures. Information on these steels may be obtained from published sources, such as API Publication 959.